Thermal Design Power

Hello All,

I've been looking into some new laptops and I've come across something that I was hoping someone could explain. Why is it that the 13" Retina Macbook Pro uses a 28w processor, while pretty much all 13" Windows Ultrabooks use 15w processors? I prefer Windows and would rather avoid using Bootcamp, but this difference is making me want to buy a Mac. Any ideas?

 

Macs are limited in other areas.


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Not the most detailed answer I've ever read...

 

Can it run at 28W 100% of the time?

Using lower power limits means thinner and lighter which lots of people prefer over higher performance and a chance of back injury from having to lift more than 2kgs.

 

I have no idea. One of the reasons I'm asking this question is because it doesn't seem to be addressed in most reviews and comparisons. Wanted to get some more info. What is clear is that the Macbook Pros definitely outperform most Windows Ultrabooks in terms of CPU performance.

 

TDP does not necessarily have any direct relation to processor performance. Some processors consume more power while at the same time having inferior performance to other processors that consume less power. It all depends on the design of the CPU. You need to be looking at benchmarks when comparing actual CPUs to get an idea of how the processor stacks up. TDP is also a rating that can be misleading because it is derived as more or less an average value. Actual power draw will go higher or lower depending on what the processor is doing.

 

I've been looking at benchmarks. The Retina Macbook Pro outperforming other laptops in benchmarks is actually what drove me to look at spec differences and then I noticed the difference in TDP.

 

Ultrabooks are probably limited (by size/cooling capabilities/hamstrung for more battery life) more so than Macbooks. But Macbooks leave a lot to be desired compared to more powerful Windows machines. What I was referring to specifically in my vague post was a PSU that can't supply enough watts to run both CPU and GPU hard without draining the battery at the same time. Or maybe the OS/serviceability/other limitations.

You have to realize you can't have skinny/light/long battery AND performance. Design (I'm a product designer) is all about tradeoffs.

 

I would qualify that remark with "not at the same time". Many of our notebooks spend most of their life plugged into a power outlet at which time more performance is desirable and some extra power consumption doesn't matter. It's a different story when running on battery when performance should be throttled back a more efficient setting such as no turbo mode on the CPU and any GPU turned off.

I would also note that most conputer manufacturers are conservative with their power supply ratings. When I moved from my Samsung NP900X4C which had a 40W PSU to the Dell Latitude E7440 I discovered that the latter locked itself down to a painfully slow CPU speed if connected to a PSU rated at less than 65W yet the internal hardware needed no more power than the Samsung notebook (confirmed by observing the power drain at the mains socket). Howevere, more recently I moved to the Latitude E7450 with an Nvidia 840M GPU which Dell supplied with a 90W PSU. I was pleasantly surprised to find that when connecting a 45W Dell PA-20 PSU (which is a conveniently portable size) I get a message about the PSU power rating and computer might needing to draw on the battery but the speed is not affected. I have also established that running a graphically demanding workload (eg a 3DMark benchmark) results in a mains socket power drain of just under 40W which is within the 45W rating but doesn't leave much spare capacity for battery charging (if needed).

John

 

BIOS generally basis power limiting on the CPU specification by reading the CPU's register for TDP and using that rather than using the laptops cooling performance whether that be better, the same or worse than the TDP spec.